Monitoring of gradient waveforms concurrently with MRI scans has been shown to be an effective means of correcting image reconstruction from data acquired in the presence of gradient waveform imperfections, eddy currents and field drifts (Barmet C et al. MRM 60(1), 2008; and Barmet C. ISMRM 2010 p. 216). For this purpose, it is known to use small magnetic field detectors exploiting the magnetic field dependence of a magnetic resonance transition. In the applications described so far, these so-called MR-type magnetic field probes are rigidly mounted in the periphery of the imaging volume. To derive the global field evolution from the signals of these probes, their positions need to be measured in a calibration step. This requirement not only extends the scan protocol but also precludes field monitoring with field probes that are subject to displacements, e.g., along with surface coil arrangements.
In many circumstances it would be highly desirable to have some kind of automatic position calibration of the magnetic field probes during actual field monitoring.
EP 0 911 642 A2 discloses a method and an apparatus for determining the location of a magnetic probe within the anatomy of a patient by means of ESR. The method includes the steps of placing an electron spin resonance sample in a known position with respect to a surgical instrument, placing the sample within the imaging reason of an MR apparatus, applying at least a first gradient magnetic field in the imaging region, and determining the resonant frequency of the sample in the presence of the gradient field. Based on the resonant frequency of the sample, the position of the sample and thus also of the surgical instrument with respect to the gradient field is then determined. To carry out a 3D position measurement, it is proposed to sequentially produce three magnetic field gradients which are mutually orthogonal and linear within the imaging region.
While EP 0 911 642 A2 is generally silent about the timing relation between the actual MR imaging measurement and the probe position measurement, it mentions one embodiment according to which the sample position is determined using three gradient fields produced by the MR apparatus.
Moreover, previously described methods such as in EP 0 911 642 A2 rely on unipolar or bipolar gradient pulses in each dimension of the position determination, which in general cannot be inserted at arbitrary positions in a given MR sequence without altering and/or disturbing and/or precluding the intended spin manipulation of the actual MR sequence. Examples for this are, for instance, (1) encoding perpendicular to the read-out plane during an echo-planar-readout, and (2) position determination during a typical spectroscopy acquisition.
Therefore, there is still a need for improved methods of determining positions of a magnetic field probe in a volume of interest within a magnetic resonance (MR) imaging or spectroscopy arrangement. In particular, it would be desirable to determine the probe position with high temporal resolution throughout and independently of an MR sequence.